Protein kinases constitute the largest family of human enzymes, encompassing well over 500 proteins. It has been found that kinases play a key role in many basic biological processes in the cell including but not limited to cell proliferation, survival, motility, morphogenesis, angiogenesis, and so on. In addition, many kinases were found involved and implicated in a number of pathological settings such as cancers, autoimmune and inflammatory diseases, eye diseases, and cardiovascular diseases. In general, kinases transmit cell-to-cell or intracellular signals by phosphorylating downstream proteins in the signal transduction pathways such that the downstream proteins are activated and thus signals can be passed from one step to the next down the signaling cascade. These signal transduction pathways are well regulated in the cell under normal physiological conditions. They are activated and shut down appropriately in response to the changes in the intra- and extracellular environments. However, in many pathological settings, one or more signal transduction pathways are often shown to be overactive and responsible for the occurrence and the progression of the diseases. Thus, blocking kinase function in disease settings by chemical or biological agents leading to the disruption of signaling pathways involved in the pathological processes could potentially disrupt or reduce the progression of the diseases and, therefore, confer clinical benefits to the relevant patients. Among many disease-related kinases, receptor tyrosine kinases c-Met (HGF/SF receptor), VEGFR2 (KDR, Flk1), PDGFRβ and c-Kit have been well characterized and considered effective targets for therapies treating diseases such as cancers, autoimmune and inflammatory diseases, and eye diseases. See, e.g., Carmeliet, P., Nature, 2005, 438:932-936; Ferrara, N. et al., Nature, 2005, 438: 967-974; Comoglio, P. M. et al., Nature Reviews: Drug Discovery, 2008, 7: 504-516.
Angiogenesis, the formation of new blood vessels from preexisting ones, plays a significant role in many pathological settings, including cancer, chronic inflammation, diabetic retinopathy, psoriasis, rheumatoid arthritis, and macular degeneration. Anti-angiogenesis therapy represents an important approach for the treatment of solid tumors and other diseases associated with dysregulated vascularization. Given a continuous string of approvals of angiogenesis inhibitor drugs such as bevacizumab, sorafenib, and sunitinib for the treatment of cancers, the clinical benefit from anti-angiogenesis therapy has become increasingly evident. See, e.g., Atkins, M. et al., Discovery, 2006, 5: 279-280; Wilhelm, S. et al., Nature Reviews: Drug Discovery, 2006, 5: 835-844.
The process of angiogenesis requires the concerted actions of multiple angiogenesis mediators as well as the participation of different cell types. Key angiogenesis mediators have been identified, including VEGF, FGF, and angiopoietin 1 and 2 (Ang1 and Ang2) that bind to their cognate receptors (VEGFRs, FGFRs and Tie1 and Tie2, respectively) expressed on endothelial cells, as well as platelet-derived growth factor (PDGF) that binds to its receptor (PDGFRα) expressed on VEGF-producing stromal cells or its receptor (PDGFRβ) expressed on pericytes and smooth muscle cells. Molecules including VEGF, FGF, PDGF, VEGFRs, FGFRs, PDGFRs, Tie1, and Tie2 are key components of multiple different signaling pathways that function in parallel to regulate angiogenesis in both physiological and clinical settings. Among these molecules, the signal transduction pathway mediated by VEGFR2 plays the most critical role in tumor angiogenesis.
A number of monoclonal antibodies (mAbs) against single angiogenesis pathway components such as VEGF and FGF have been developed to block angiogenesis and shown to slow down tumor growth in preclinical and clinical studies. However, to a linear pathway, targeting a single component of the pathway is less effective than simultaneous blocking multiple components of the pathway. Thus development of multiplex small molecular kinase inhibitors is desirable for achieving more efficient angiogenesis inhibition. Since VEGFR2 and PDGFRβ are targeted by both sorafenib and sunitinib, the clinical benefits demonstrated in the use of both drugs unambiguously validate VEGFR2 and/or PDGFRβ kinase as effective target in the treatment of diseases such as cancer. See, e.g., Atkins, M. et al., supra; Wilhelm, S. et al., supra.
The c-Kit proto-oncogene, also known as KIT, CD-117, stem cell factor receptor, or mast cell growth factor receptor, is a receptor tyrosine kinase and a member of the split kinase domain subfamily. Activation of c-Kit by its natural ligand, stem cell factor (SCF), promotes receptor dimerization and autophosphorylation at tyrosine residues Tyr567 and Tyr719. See, e.g., Chian R. et al, Blood, 2001, 98: 1365-1373. Signaling through c-Kit plays an important role in cellular transformation and differentiation, including proliferation, survival, adhesion, and chemotaxis. See, e.g., Linnekin D., Int. J. Biochem. Cell Biol., 1999, 31: 1053-1074. C-Kit expression has been reported in a wide variety of human malignancies such as small cell lung cancer (SCLC), gastrointestinal stromal tumors (GIST), colorectal cancer and so on. As a well proven target in cancer therapy, c-Kit inhibitor such as Gleevec® has been used to treat CML, GIST, and other cancers.
C-Met tyrosine kinase is a cell surface receptor normally activated by its natural ligand, hepatocyte growth factor/scatter factor (HGF/SF). Upon HGF binding, c-Met protein is activated by autophosphorylation and recruits downstream effectors to its cytoplasmic domain. The resulting multi-protein signaling complex can in turn activate a number of downstream intracellular signaling events in epithelial cells and lead to a wide range of cellular responses including but not limited to proliferation, survival, angiogenesis, wound healing, tissue regeneration, scattering, motility and invasion. See, e.g., Comoglio, P. M. et al., supra; and Benvenuti, S, and Comoglio, P. M., J. Cellular Physiology, 2007, 213: 316-325.
Numerous evidences have implicated c-Met as one of the leading molecular targets in cancer therapy. See, e.g., Knudsen, B. S. et al., Current Opinion in Genetics & Development, 2008, 18: 87-96. C-Met has been implicated as a proto-oncogene, which is found genomically amplified, over-expressed, mutated, or aberrantly activated in many types of cancers, suggesting its roles in the tumor growth, invasiveness and metastasis. In addition, elevated c-Met activation has been found in solid tumors which develop resistance to anti-EGFR therapies during the course of treatment, implicating a compensatory role of c-Met activation to the EGFR signaling pathway (see, e.g., Smolen, G. A et al., Proc. Natl. Acad. Sci. USA, 2006, 103: 2316-2321; Lutterbach, B. et al., Cancer Res., 2007, 67: 2081-2088). Thus inhibition of c-Met signaling is considered as a potentially effective therapeutic strategy against solid tumors whose growth is wholly or partially c-Met driven (see, e.g., Smolen, G. A et al., supra). It is thus pharmacologically preferable to develop small molecule kinase inhibitors against c-Met for the treatment of cancer.
Studies have reported that anti-angiogenesis therapies can lead to increased local invasion and distal metastasis of tumor cells and thus elicit malignant progression of tumors (see, e.g., Ebos, J. M. L. et al., Cancer Cell, 2009, 15: 232-239; Paez-Ribes, M. et al., Cancer Cell, 2009, 15: 220-231). This unexpected yet important finding calls for a new generation of anti-angiogenesis therapies which can not only disrupt tumor angiogenesis and arrest tumor growth but also are able to prevent tumor invasion and metastasis at the same time. Due to the important roles that c-Met plays in tumor invasiveness and metastasis, it is strongly conceivable that simultaneous inhibition of VEGF/VEGFR2 and c-Met signaling pathways will produce better clinical outcomes than the current generation of anti-angiogenesis therapy in solid tumor treatment (see, e.g., Loges, S. et al., Cancer Cell, 2009, 15: 167-170).
This invention provides a solution by combining two anti-tumor therapeutic mechanisms with small molecule drugs targeting one or more protein kinases (e.g., both VEGFR2 and c-Met), which offer unexpected clinical advantages over the currently available anti-angiogenesis therapeutics.